1. Field of the Invention
The present invention relates to an image forming apparatus that forms an image on a recording medium using electrophotographic technology, such as a copying machine, a printer, or a facsimile machine, and more specifically, it relates to a type of image forming apparatus that transfers an image to a recording medium in a nip between an image bearing member and a conveying belt, and a sheet conveying method.
2. Description of the Related Art
An electrophotographic image forming apparatus is configured to transfer an image formed on an image bearing member to a recording medium in a transfer portion, to convey the recording medium to a fixing device, where the image is fixed, and then to output the recording medium. In such an image forming apparatus, in order to transfer the image to the recording medium without causing defective transfer, it is necessary to stably convey the leading edge of the recording medium when conveying the recording medium to the transfer portion.
For example, if there is an upward curl (a curl that is convex downward) at the leading edge of the recording medium, the upward curl is flattened when the leading edge of the recording medium enters the transfer portion, and the leading edge of the recording medium is thereby slightly displaced in the conveying direction. As a result, the leading edge of the recording medium rubs against the image formed on the image bearing member, and therefore the image may be disrupted.
So, in order to stably convey the leading edge of the recording medium to the transfer portion, a technique is proposed in which a conveying belt that attracts and conveys a recording medium is disposed opposite to an image bearing member, and a transfer nip where an image is transferred to the recording medium is formed between the conveying belt and the image bearing member. This technique is described in Japanese Patent Laid-Open No. 2000-185839. In this configuration, the recording medium is conveyed to the transfer nip while being attracted to the conveying belt, and an image formed on the image bearing member is transferred to the recording medium in the transfer nip. As described above, by attracting the recording medium to the conveying belt, the curled leading edge of the recording medium can be flattened along the conveying belt, and the leading edge can be stably conveyed to the transfer nip.
However, the conventional image forming apparatus in which the conveying belt is used in the transfer portion has the following problems. This image forming apparatus can stably convey the recording medium to the transfer nip by attracting the curled leading edge of the recording medium to the conveying belt. However, when a curl is formed at the leading edge of the recording medium, the position of the leading edge of the recording medium at the time when the leading edge of the recording medium is attracted into contact with the conveying belt may vary.
For example, if there is a downward curl (a curl that is convex upward) at the leading edge of the recording medium to be conveyed to the conveying belt, the site where the leading edge of the recording medium lands on the conveying belt is slightly displaced upstream in the conveying direction compared to the case where there is no curl. The recording medium is conveyed to the transfer nip with the displacement uncorrected. The larger the size of curl, the larger the displacement.
The conveying belt 45 rotates at the same speed as the image bearing member rotating at a constant speed (process speed). Therefore, once the position of the leading edge of the recording medium is displaced, the displacement cannot be corrected.
If the position of the leading edge of the recording medium relative to the conveying belt varies depending on the size of curl as described above, the position of the image relative to the recording medium varies when the image is transferred to the recording medium in the transfer nip, and image displacement occurs.